Preharvest Application of Melatonin Affects the Color, Strength, and Antioxidant Capacity of Pear Peels by Regulating Phenylpropane Metabolism

Melatonin is an important regulator of fruit growth and development. To explore the physiological mechanism whereby preharvest melatonin application regulates the polyphenol content of 'Yuluxiang' pear peel, we sprayed 0.1 mM melatonin during the first fruit expansion and early color change periods, and the control group were sprinkled with fresh water. Then, we measured the contents of anthocyanin, lignin, and major monomeric phenolics and the activities of key enzymes associated with phenolic metabolism. The results showed that melatonin application significantly increased the content of total phenolics, total flavonoids, total anthocyanins, and lignin in the peel from the color change to mature development stages. Near maturity, the activities of all key enzymes, except dihydroflavonol-4-reductase, were higher than those in the control samples, but significant differences in enzyme activity occurred at different time points. Compared with the control group, the fruit peels of the melatonin-treated plants exhibited a higher antioxidant activity and accumulated more flavonols. Thus, preharvest spraying of melatonin can alter the activity of key enzymes associated with phenolic metabolism, increasing the total phenol, flavonoid, anthocyanin, and lignin contents, which in turn, affects the color, strength, and antioxidant capacity of pear peels.